Hybrid electric vehicles and the like are equipped with an electric drive system in addition to a conventional engine. This electric drive system includes a battery which is a DC power supply, an inverter device which is a power conversion device, and an AC motor which is a motor. In a vehicle including this electric drive system, DC power supplied from the battery is converted to AC power by the inverter device, and this AC power is used to drive the AC motor, thereby obtaining driving power of the vehicle.
FIG. 14 is a circuit configuration diagram of a common electric drive system mounted on a vehicle. As described above, the electric drive system includes a battery 1, an inverter device converting DC power obtained from battery 1 to AC power, and an AC motor 8 driven by the AC power output from the inverter device.
The inverter device includes a switching module 2 having switching elements 6, a switching control circuit (not shown) controlling switching elements 6, a smoothing capacitor 3 smoothing the DC power supplied to switching elements 6, and busbars 4, 5 electrically connecting switching module 2 and smoothing capacitor 3 to each other.
For switching element 6, an IGBT (Insulated Gate Bipolar Transistor), a power MOSFET (Metal Oxide Semiconductor-Field Effect Transistor) or the like is used. In addition, the switching control circuit causes ON/OFF operation of switching elements 6 by PWM (Pulse Width Modulation) control in response to a signal from a not-shown ECU (Electrical Control Unit), to operate the inverter device under optimum conditions in accordance with a running state of the vehicle.
Examples of switching module 2 include a switching module provided with the above switching control circuit outside a housing thereof, and a switching module called IPMs (Integrated Power Modules) where the switching control circuit is incorporated inside the housing. In recent years, the IPMs have become mainstream with growing demand for improved ease of assembly and smaller inverter devices.
As shown in FIG. 14, P and N terminals which are a positive electrode and a negative electrode of switching module 2 are electrically connected to a positive electrode and a negative electrode of battery 1, respectively. Further, between switching module 2 and battery 1, smoothing capacitor 3 for smoothing the DC power supplied from battery 1 is connected in parallel.
For connecting lines that connect the P terminal and the N terminal of switching module 2 to a positive electrode and a negative electrode of smoothing capacitor 3, respectively, busbars 4, 5 which are plate-like bodies made from a conductive material are used. On the other hand, U, V, W terminals which are terminals on an output side of switching module 2 are electrically connected to respective input terminals of three-phase AC motor 8.
Smoothing capacitor 3 is required to have a very large capacity. For smoothing capacitor 3 used in hybrid electric vehicles in practical use, a capacitor having a capacity of several hundred μF to several thousand g has been used. Smoothing capacitor 3 is accordingly a very large capacitor, and is thus usually installed outside the housing of switching module 2.
In the above inverter device, switching of switching elements 6 is performed at high speed, for example, at 10 kHz. Because of this, line inductance of internal lines of switching module 2 and busbars 4, 5 are unignorable.
Stray inductance due to these connecting lines causes application of a large surge voltage to switching elements 6 during switching. Although no problem arises when switching element 6 has a withstand voltage set sufficiently high with respect to a power supply voltage, this surge voltage may break switching element 6 without sufficient margin.
In order to reduce the line inductance, therefore, it is necessary to dispose smoothing capacitor 3 and switching module 2 in proximity to each other, so that the connecting lines can be shortened and thus the line inductance is reduced. Further, the line inductance may also be reduced by disposing a to-line and a from-line of the connecting lines in proximity to each other. This is because by disposing the to-line and the from-line close to each other, mutual inductance reduces the line inductance.
This measure alone, however, cannot completely suppress generation of a surge voltage. There is thus an inverter device configured to have a separately provided circuit for protecting switching elements 6, which is called a snubber circuit, thereby absorbing a surge voltage generated inside the circuit to prevent the surge voltage from being applied to switching elements 6.
Japanese Patent Laying-Open No. 2003-319665 discloses an inverter device in which a snubber circuit is constituted of busbars.
FIG. 15 is a schematic perspective view showing the structure of the inverter device in Japanese Patent Laying-Open No. 2003-319665. As illustrated, smoothing capacitor 3 and switching module 2 are connected to each other by a pair of busbars 15, 16. Switching module 2 is contained in the housing.
Although not illustrated, the pair of busbars 15, 16 is further connected to battery 1. Further, smoothing capacitor 3 is disposed as being divided into three parts due to its very large capacity, and the capacitors are connected to switching module 2 in parallel.
The pair of busbars 15, 16 has, in a region where switching module 2 is connected to smoothing capacitor 3, a facing region where their main surfaces are disposed to face each other and to be in proximity to each other. This facing region is provided for reducing the line inductance discussed above. In this facing region, a high dielectric 9 is provided between the pair of busbars 15 and 16.
High dielectric 9 is interposed between the pair of busbars 15 and 16 to form a capacitor. This capacitor forms a snubber circuit.
By providing this snubber circuit, the surge voltage applied to switching elements 6 during switching operation is absorbed, thereby suppressing the breakdown of switching elements 6.
Moreover, Japanese Patent Laying-Open No. 2004-312925 discloses electric equipment in which a smoothing capacitor and an inverter circuit are provided inside a single housing.
Japanese Patent Laying-Open No. 2005-20953 discloses a resin-molded busbar.